1. Field of the Invention
The present invention relates to a liquid crystal display device and a method of manufacturing the same.
2. Description of the Related Art
Recently, regarding a thin display device such as a liquid crystal display device, there is a demand for a high image quality, price cutting, and power saving. Regarding a color filter for the liquid crystal display device, there is a need to be improved in order to achieve a high image quality, such as sufficient chromatic purity, high contrast, smoothness (flatness), and the like.
As to the liquid crystal display device, alignment methods of liquid crystals, such as Vertically Alignment (VA), Hybrid-aligned Nematic (HAN), Twisted Nematic (TN), Optically Compensated Bend (OCB), and Continuous Pinwheel Alignment (CPA), and driving methods of the liquid crystals are proposed for a high image quality. With these technologies, the liquid crystal display devices having a wide viewing angle and a high-speed response have been put to practical use.
In the VA system, a variety of improved modes have been developed, for example Multi-Domain Vertically Alignment (MVA), Patterned Vertically Alignment (PVA), Vertically Alignment Electrically Controlled Birefringence (VAECB), Vertical Alignment Hybrid-aligned Nematic (VAHAN), and Vertically Alignment Twisted Nematic (VATN). Moreover, in the longitudinal electric field type liquid crystal display device of the VA system or the like in which a driving voltage is applied in the direction of thickness of the liquid crystal layer, a rapider liquid crystal response, a wide viewing angle technology, and higher transmittance are demanded.
The liquid crystal molecules are initially aligned to be perpendicular to the surface of a substrate and it is difficult to determine the inclination direction of the liquid crystal molecules at the time when a voltage is applied. Therefore, in the MVA technology, in order to remove the problem in which the vertically aligned liquid crystal molecules become unstable at the time when a driving voltage of the liquid crystal is applied, a plurality of rib-shaped convex portions are provided, and a plurality of liquid crystal domains are formed such that each domain has an alignment direction different from each other between the ribs, which leads to a wide viewing angle.
Patent Literature 1 (Japanese Patent No. 3957430) discloses a technology in which the liquid crystal domains are formed by using first and second alignment control structures (ribs).
Patent Literature 2 (Jpn. Pat. Appln. KOKAI Publication No. 2008-181139) discloses a technology in which four liquid crystal domains are formed by using an optical alignment method. In Patent Literature 2, it is described that, in each liquid crystal domain, aligning processing related to the strict control on a pre-tilt angle (89 degrees from the horizontal direction) are necessarily performed multiple times (four times) to secure a wide viewing angle, and alignment axes at an angle of 90 degrees from each other are necessary.
Patent Literature 3 (Japanese Patent No. 2859093) and Patent Literature 4 (Japanese Patent No. 4364332) disclose a technology in which vertically aligned liquid crystal molecules are controlled with an oblique electric field by using a first electrode (hereinafter, also referred to as a counter electrode, a transparent electrode, a display electrode, or a common electrode on a color filter side) which is formed of a transparent conductive film on a color filter substrate side, and also using a first electrode (also, referred to as a pixel electrode) and a second electrode (also, referred to as a common electrode on an array substrate side) which are formed on the array substrate side. In Patent Literature 3, it is described that liquid crystal with negative dielectric anisotropy is used, and in Patent Literature 4, it is described that the liquid crystal with positive dielectric anisotropy is used. In Patent Literature 4, there is no description about the liquid crystal with negative dielectric anisotropy.
In the basic configuration of the liquid crystal display device of the VA system, the TN system, or the like, a liquid crystal layer is interposed between a color filter substrate with a third electrode and an array substrate with a plurality of first electrodes that are provided for driving liquid crystal. For instance, the first electrode is a transparent electrode that is electrically connected with a thin film transistor (TFT) element and is formed in a comb-teeth shape pattern. In this typical configuration, a driving voltage is applied between the third electrode on the color filter and the first electrodes formed on the array substrate side, and the liquid crystal is driven by application of this driving voltage. The transparent conductive film that forms the first electrodes and the third electrode are usually a thin film made of a conductive metallic oxide such as Indium Tin Oxide (ITO: a metallic oxide thin film of indium tin) or Indium Zinc Oxide (IZO).
Patent Literature 5 (Jpn. Pat. Appln. KOKAI Publication No. 2009-92815) discloses a liquid crystal display device that uses a pair of substrates with different pre-tilt angles.
Patent Literature 6 (Jpn. Pat. Appln. KOKAI Publication No. 2010-217867) describes an aligning agent used for an alignment film.
As described above, in the MVA technology applied to the liquid crystal display device of the vertical alignment system, the liquid crystal domain is formed by using the alignment control structure called a rib (or ribs) to secure a wide viewing angle. When the liquid crystal molecules have negative dielectric anisotropy, the liquid crystal molecules located between two ribs structures made of a resin formed on the color filter or the like are aligned perpendicular to the surface of a substrate before application of a driving voltage, and tend to incline horizontally in a perpendicular direction to the two ribs during the application of the driving voltage. However, although the voltage is applied, the inclining directions of the liquid crystal molecules at the center of a space between the two ribs are not determined to be equal, but the liquid crystal molecules are aligned in a spray alignment or a bend alignment. The disturbance of liquid crystal molecular alignment leads to the roughness in liquid crystal display, display irregularity, and deterioration of transmittance.
Moreover, in the case of MVA system, it is difficult to finely control the inclination of the liquid crystal molecules with the driving voltage, and to control the halftone display. Particularly, in the MVA system, the linearity in the relation between the display (response speed) and the driving voltage is low, and it is difficult to display the halftone with a low driving voltage.
As a technique of improving the halftone display, as disclosed in Patent Literature 3 and Patent Literature 4, a technique that controls the liquid-crystal molecular alignment with a method of using the oblique electric field by using first, second and third electrodes is effective. In the method of using the oblique electric field, the inclining direction of the liquid crystal molecules can be set. Moreover, with the method of using the oblique electric field, the inclining amount of the liquid crystal molecules can be easily controlled, and the halftone display can be controlled effectively.
However, even if the oblique electric field method is used, measures to resolve disclination of the liquid crystal might not be sufficient. Disclination means occurrence of an event that a pixel has areas where the transmittance of light is different due to the unintended alignment disturbance or non-alignment of the liquid crystals.
Patent Literature 3 discloses that the disclination at the center of the pixel is fixed by providing an alignment control window where the transparent conductive film is not present in the third electrode at the center of the pixel. However, Patent Literature 3 does not disclose a method of resolving the disclination at the periphery of the pixel. Moreover, though the disclination can be fixed at the center of the pixel, a method of minimizing the disclination is not disclosed. In addition, a technology which improves the response of the liquid crystal is not discussed.
In Patent Literature 4, a dielectric layer is stacked on a transparent conductive film (transparent electrode). Accordingly, the effect of the oblique electric field method is increased, so the technology is preferable. However, as illustrated in FIG. 7 of Patent Literature 4, the liquid crystals of the vertical alignment might remain at the center of the pixel and the edge of the pixel even after the voltage is applied, which sometimes lead to decreases in transmittance and aperture ratio. In Patent Literature 4, the liquid crystal with negative dielectric anisotropy is not discussed. When the liquid crystal with positive dielectric anisotropy is used, it is difficult to improve the transmittance due to the disclination at the center of the pixel. Therefore, it is difficult to adopt the technology of Patent Literature 4 in a transflective liquid crystal display device.
Patent Literature 5 discloses a liquid crystal display device using a pair of substrates with different pre-tilt angles. In Patent Literature 5 (especially, claim 4, FIG. 3, and Paragraph 0024, and the like), a so-called Polymer Sustained Alignment (PSA) technology is disclosed which adds a monomer, which is polymerized with ultraviolet light irradiation, to a liquid crystal layer to cause the liquid crystal layer to turn into liquid crystal cells, and irradiates the ultraviolet light while applying a voltage to form an aligned film. Moreover, in Paragraph 0035 of Patent Literature 5, it is disclosed that the pre-tilt angle given to the liquid crystal molecules on the TFT substrate side is set to 1° or greater and 4° or less from the perpendicular direction to the substrate, and the pre-tilt angle in the color filter substrate side is 0°. Moreover, in Paragraph 0048 of Patent Literature 5, a configuration of a color filter is disclosed in which an ITO electrode is arranged in the color filter, and a vertical alignment film is further coated thereon. In FIGS. 4 to 6, and Paragraph 0022 of Patent Literature 5, a sectional configuration of a pixel is disclosed.
However, Patent Literature 5 does not disclose a technology in which domains of 2 or 4 liquid-crystal molecular alignments are formed in a unit pixel (one pixel) of a liquid crystal display device, and the liquid crystal molecules are arranged in one direction in one domain. When the liquid crystal molecules are homogeneously aligned in one domain, it is difficult to secure high transmittance in the liquid crystal display device. With the liquid crystal arrangement in FIGS. 4 to 6 of Patent Literature 5, it is difficult to achieve high transmittance because the alignments of the liquid crystal molecules in the unit pixel are not uniform. In Paragraph 0031 of Patent Literature 5, even though the ultraviolet light is irradiated from the color filter substrate side, the transmittance for the ultraviolet light is different among red, green, blue, and the like of the color filters. Therefore, there might be a difference in the pre-tilt angle of the liquid crystal among different colors, which may result in a difference in the response of the liquid crystal among different colors. In addition, in Patent Literature 5, the following are not discussed: a TFT light-shielding unit, a black matrix of the color filter, the presence of a monomer which is left unpolymerized due to the blocking of the ultraviolet ray in the pixels with low transmittance for the ultraviolet ray, the decrease in reliability of the alignment film attributable to insufficient curing. In the PSA technique which adds a photopolymerizable monomer to the liquid crystal for causing optical polymerization, a change in the liquid crystal response or an image sticking occurs over time due to the presence of the monomer which remains unpolymerized, or the insufficiently cured alignment film.